This invention relates to uninterruptible power supply (UPS) systems, and more particularly toward UPS systems that include batteries such as conventional lead-acid batteries as a standby power source.
UPS systems are often installed in environments in which continuous operation is critical, even in the event of a loss of main power. For example, such systems may be installed in airports, hospitals, processing plants and computer centers. In each case, a total loss of power may lead to catastrophic results (e.g., a loss of power in the middle of surgery may result in the death of the patient).
In typical UPS systems, circuitry is provided that monitors power being supplied from a main source of power, often via a connection to a DC buss. A bank of batteries, often lead-acid batteries, is connected to a DC buss that feeds the critical load to provide temporary power as soon as the voltage on the buss drops below battery voltage. The batteries are usually intended to provide temporary power only until a standby power source such as a backup diesel generator ("GENSET") can be brought on-line, or in some instances, are the sole backup power source (e.g., providing enough backup power to permit a normal, sequential shutdown of a processing system). The batteries, therefore, typically provide power for a very short time, until the standby generator is running at full speed and providing backup power.
One deficiency of traditional battery based UPS systems is that the battery life is often significantly reduced due to power disturbances that are very short in duration (e.g., less than a few seconds). It is generally known that approximately ninety percent of the power disturbances that occur are of the very short duration type (for example, some installations are known to have very short duration outages which occur hundreds of times each month). These short duration outages, however, still cause the line voltage to sag, leading to the batteries experiencing a discharge/charge cycle for each outage. The life of the batteries, as is well known, is inversely proportional to the number of discharge/charge cycles that the batteries are subjected to. Thus, even very short duration outages may have a severe negative impact on battery life. Moreover, it is also known that AC ripple is present on the DC buss that the batteries are connected to. This ripple further accelerates the loss of useful life of the batteries.
In view of the foregoing, it is an object of the present invention to provide an improved uninterruptible power supply in which batteries are protected from electrical disturbances that artificially reduce battery life.
It is also an object of the present invention to provide an improved method of supplying battery power to an uninterruptible power supply so that useful battery life is extended over conventional supplies.